Experimental measurement of moisture distribution in the adhesive layer using near-infrared spectroscopy

Adhesive joints are degraded when exposed to moist conditions, making it essential to assess the permeation distance of absorbed moisture. However, monitoring the water penetration in the adhesive layer is challenging because the adhesive layer is sandwiched between the adherends. To overcome this difficulty, we proposed a new monitoring system using fiber-type near-infrared spectroscopy (NIRS), and the moisture distribution at the joint was successfully observed by introducing quartz glass as one side of the substrate. First, the change in the absorbance spectra owing to water absorption by an epoxy adhesive was investigated by immersing epoxy-coated aluminum alloy plates in temperature-controlled water. It was found that the water content can be evaluated by monitoring the change in the peak intensity appearing around 1900-1950 nm. Next, water penetration into the adhesive layer sandwiched between the quartz glass and aluminum alloy was investigated by scanning the surface. The water permeation distance was found to change with temperature. Under the 23 degrees C immersion condition, approximately 2.5 mm of water penetration was observed after 8 weeks. Because the simulation results using Fick's law of diffusion showed similar trends, it was experimentally confirmed that Fickian diffusion dominates the water distribution in the adhesive layer.